wwtest: native package-test coordinator

Single-directory package-test planner: discovers *.ww, groups white-box
and external <pkg>_test sources, composes the combined test package,
builds it through the sibling ww driver, and runs it with the runtime's
-package/-list/-timeout-ms contract. test/package holds its E2E corpus
and the manual runtime fixture set.

test/package/dependency/dep/deptest.ww

@@ -0,0 +1,7 @@
+package dep;
+
+// Noncanonical migration spelling: the actual line-leading @test makes this
+// test-only.  It must be excluded when dep is imported by the root package.
+@test fn imported_dependency_test_must_not_leak() void = {
+	assert(false);
+};